Capacitive physical quantity sensor

Abstract

A capacitive physical quantity sensor includes a sensor element and a detecting element. The sensor element includes first and second fixed electrodes facing a movable electrode. A first voltage is applied to the first fixed electrode and a second voltage is applied to the second fixed electrode. The detecting circuit includes a capacitance-voltage conversion circuit, in which an operational amplifier, a capacitor and a switch including a P-channel MOS transistor and a N-channel MOS transistor are disposed. The transistors have a back gate potential, which is approximately equal to an average voltage of the first voltage and the second voltage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:

FIG. 1 is a block diagram showing a capacitive physical quantity sensor according to a first embodiment;

FIG. 2 is a circuit diagram showing a switch in a C-V conversion circuit shown in FIG. 1;

FIG. 3 is a timing chart for detecting an acceleration;

FIG. 4A is a circuit diagram showing a sampling and holding circuit in a capacitive physical quantity sensor according to a second embodiment, and FIG. 4B is a circuit diagram showing a switch in the sampling and holding circuit; and

FIG. 5 is a circuit diagram showing a switch in a C-V conversion circuit of a conventional capacitive physical quantity sensor.

Claims

1. A capacitive physical quantity sensor comprising: a sensor element including a movable electrode displaceable in accordance with a variation of a physical quantity, and first and second fixed electrodes facing the movable electrode, wherein a first periodic carrier wave voltage is applied to the first fixed electrode and a second periodic carrier wave voltage is applied to the second fixed electrode, the first and second periodic carrier wave voltages are inverse to each other; anda detecting circuit including a capacitance-voltage conversion circuit, into which an input potential corresponding to a variation of a capacitance between the movable electrode and the first and second fixed electrodes is input, and from which a voltage signal corresponding to the variation of the capacitance is output, whereinthe capacitance-voltage conversion circuit includes: an operational amplifier, wherein a signal corresponding to the variation of the capacitance is input into an inverting input terminal of the operational amplifier;a capacitor connected between the inverting input terminal and an output terminal of the operational amplifier; anda switch, which is connected in parallel to the capacitor, including a P-channel MOS transistor and a N-channel MOS transistor, which are connected in parallel to the capacitor, andthe P-channel MOS transistor and the N-channel MOS transistor have a back gate potential, which is approximately equal to an average voltage of the first periodic carrier wave voltage and the second periodic carrier wave voltage.

2. The capacitive physical quantity sensor according to claim 1, wherein: the detecting circuit further includes a sample-hold circuit for sampling and holding the voltage signal from the capacitance-voltage conversion circuit,the sample-hold circuit includes a switch for switching the sampling and the holding,the switch includes a P-channel MOS transistor and a N-channel MOS transistor, which are connected in parallel to each other, andthe P-channel MOS transistor and the N-channel MOS transistor have a back gate potential, which is approximately equal to the average voltage.

3. The capacitive physical quantity sensor according to claim 1, wherein: the detecting circuit further includes a LPF circuit for filtering a frequency band in a predetermined range from the voltage signal of the capacitance-voltage conversion circuit,the LPF circuit includes a switch,the switch includes a P-channel MOS transistor and a N-channel MOS transistor, which are connected in parallel to each other, andthe P-channel MOS transistor and the N-channel MOS transistor have a back gate potential, which is approximately equal to the average voltage.

4. The capacitive physical quantity sensor according to claim 1, wherein: the physical quantity is an acceleration.

5. The capacitive physical quantity sensor according to claim 1, wherein: the first and second periodic carrier wave voltages have rectangular-wave shapes, and vary between 0V and a predetermined voltage.

6. The capacitive physical quantity sensor according to claim 5, wherein: the second periodic carrier wave voltage is set to 0V, when the first periodic carrier wave voltage is set to the predetermined voltage; andthe second periodic carrier wave voltage is set to the predetermined voltage, when the first periodic carrier wave voltage is set to 0V.

7. The capacitive physical quantity sensor according to claim 1, wherein: the back gate potential is constant to be the average voltage.

8. A capacitive physical quantity sensor comprising: a sensor element including a movable electrode displaceable in accordance with a variation of a physical quantity, and first and second fixed electrodes facing the movable electrode, wherein a first periodic carrier wave voltage is applied to the first fixed electrode and a second periodic carrier wave voltage is applied to the second fixed electrode, the first and second periodic carrier wave voltages are inverse to each other; anda detecting circuit including a capacitance-voltage conversion circuit, into which an input potential corresponding to a variation of a capacitance between the movable electrode and the first and second fixed electrodes is input, and from which a voltage signal corresponding to the variation of the capacitance is output, whereinthe detecting circuit further includes a sample-hold circuit for sampling and holding the voltage signal,the sample-hold circuit includes a switch for switching the sampling and the holding,the switch includes a P-channel MOS transistor and a N-channel MOS transistor, which are connected in parallel to each other, andthe P-channel MOS transistor and the N-channel MOS transistor have a back gate potential, which is approximately equal to an average voltage of the first periodic carrier wave voltage and the second periodic carrier wave voltage.

9. The capacitive physical quantity sensor according to claim 8, wherein: the detecting circuit further includes a LPF circuit for filtering a frequency band in a predetermined range from the voltage signal of the capacitance-voltage conversion circuit,the LPF circuit includes a switch,the switch includes a P-channel MOS transistor and a N-channel MOS transistor, which are connected in parallel to each other, andthe P-channel MOS transistor and the N-channel MOS transistor have a back gate potential, which is approximately equal to the average voltage.

10. The capacitive physical quantity sensor according to claim 8, wherein: the physical quantity is an acceleration.

11. The capacitive physical quantity sensor according to claim 8, wherein: the first and second periodic carrier wave voltages have rectangular-wave shapes, and vary between 0V and a predetermined voltage.

12. The capacitive physical quantity sensor according to claim 11, wherein: the second periodic carrier wave voltage is set to 0V, when the first periodic carrier wave voltage is set to the predetermined voltage; andthe second periodic carrier wave voltage is set to the predetermined voltage, when the first periodic carrier wave voltage is set to 0V.

13. The capacitive physical quantity sensor according to claim 8, wherein: the back gate potential is constant to be the average voltage.

14. A capacitive physical quantity sensor comprising: a sensor element including a movable electrode displaceable in accordance with a variation of a physical quantity, and first and second fixed electrodes facing the movable electrode, wherein a first periodic carrier wave voltage is applied to the first fixed electrode and a second periodic carrier wave voltage is applied to the second fixed electrode, the first and second periodic carrier wave voltages are inverse to each other; anda detecting circuit including a capacitance-voltage conversion circuit, into which an input potential corresponding to a variation of a capacitance between the movable electrode and the first and second fixed electrodes is input, and from which a voltage signal corresponding to the variation of the capacitance is output, whereinthe detecting circuit further includes a LPF circuit for filtering a frequency band in a predetermined range from the voltage signal of the capacitance-voltage conversion circuit,the LPF circuit includes a switch,the switch includes a P-channel MOS transistor and a N-channel MOS transistor, which are connected in parallel to each other, andthe P-channel MOS transistor and the N-channel MOS transistor have a back gate potential, which is approximately equal to an average voltage of the first periodic carrier wave voltage and the second periodic carrier wave voltage.

15. The capacitive physical quantity sensor according to claim 14, wherein: the physical quantity is an acceleration.

16. The capacitive physical quantity sensor according to claim 14, wherein: the first and second periodic carrier wave voltages have rectangular-wave shapes, and vary between 0V and a predetermined voltage.

17. The capacitive physical quantity sensor according to claim 16, wherein: the second periodic carrier wave voltage is set to 0V, when the first periodic carrier wave voltage is set to the predetermined voltage; andthe second periodic carrier wave voltage is set to the predetermined voltage, when the first periodic carrier wave voltage is set to 0V.

18. The capacitive physical quantity sensor according to claim 14, wherein: the back gate potential is constant to be the average voltage.